Electrolytic production of sulfonic acids from condensed ring aromatic hydrocarbons



United States Patent 3,214,356 ELECTROLYTIC PRODUCTION OF SULFONIC ACIDS FROM CONDENSED RING ARQMATIC HYDROCARBONS Junior W. Loveland, Swarthmore, Pa., assignor to Sun ;)il Company, Philadelphia, Pa., a corporation of New ersey No Drawing. Filed Mar. 29, 1963, Ser. No. 269,191 8 Claims. (Cl. 204-72) This invention provides a method for preparing sulfonic acids from certain types of condensed ring aromatic hydrocarbons. The method of the invention involves cathodic reduction, i.e., electrolysis, of the aromatic hydrocarbon to produce a free radical which subsequently reacts in situ with sulfur trioxide to produce a sulfonic acid anion. The latter subsequently reacts in situ with hydrogen ion to produce an aromatic sulfonic acid. By way of example, 9,10-dihydroanthracene-9,lO-disulfonic acid and 9,10-dihydroanthracene-9-monosulfonic acid can be prepared from anthracene. Likewise, 1,4-dihydronaphthalene-1,4-disulfonic acid and 1,4dihydronaphthalene-1- disulfonic acid can be prepared according to the invention from naphthalene. Such. dihydrodisulfonic acids and dihydromonosulfonic acids are useful both as wetting agents and as starting materials for the preparation of azo dye coupling agents.

Sulfonic acids are obtained from an aromatic hydrocarbon of the type hereinafter specified by electrolyzing the aromatic hydrocarbon, at the cathode of an electrolytic cell, at a potential greater than its half-wave potential, i.e., by cathodic reduction, in the presence of (1) an electrolyte, (2) a solvent, and (3) S0 and by reacting in situ the sulfonic acid anion thereby formed with hydrogen ion. The half-wave potential is a property which is determined polarographically according to the procedure described for example, in Kirk and Othmer, Encyclopedia of Chemical Technology, volume 10, pp. 886890 (1947). It is the potential of the inflection point of a currentpotential diagram obtained under the described conditions. It is a measure of the potential at which electrolytic reaction, usually reduction, takes place in the material in question. The electrolytic reaction which occurs at the cathode is one of reduction and the potential required to effect same is generally negative. The electrolytic reaction which occurs at the anode is one of oxidation, and the potential required to effect same is generally positive. All half-wave potentials referred to herein are cathodic half-wave potentials and are, in addition, all expressed as potentials relative to a calomel reference electrode.

The products obtained according to the invention may be monosulfonic acids or disulfonic acids. Typical reactions which occur in the method of the invention are as follows, using anthracene as an example of the starting material.

+ 2 electrons ice The aromatic compounds which can be converted to sulfonic acids by the method of the invention have certain characteristics. As stated, they are hydrocarbons. In addition, they contain two or more fused, i.e., condensed, aromatic rings. Examples of such aromatic hydrocarbons are naphthalene, anthracene, phenanthrene, methylnaphthalene, etc. Aromatic hydrocarbons such as biphenyl are not suitable, for although they contain two or more aromatic rings, they do not contain at least two condensed aromatic rings. Likewise, tetralin is unsuitable, for although it contains at least two condensed rings, it does not contain two condensed aromatic rings. Benzene is obviously not within the above definition of a suitable starting material. In addition to containing at least two condensed aromatic rings, the aromatic hydrocarbons suitable for the present purpose are further characterized in that they contain a condensed aromatic ring which has at least two hydrogen atoms attached to nuclear carbon atoms. Examples of aromatic hydrocarbons which meet these criteria are naphthalene, anthracene, 1,2,3,4,5,6,7,8-octamethylanthracene, etc. A compound such as octamethylnaphthalene does not meet this requirement because it has no hydrogen atoms attached to nuclear carbon atoms of a condensed aromatic ring. A

compound such as 1,2,3,6,7,8-hexamethylnaphthalene is unsuitable because although it has two hydrogen atoms attached to nuclear carbon atoms of condensed aromatic rings, they are not on the same condensed ring.

The cathodic reduction of the invention is carried out in the presence of an electrolyte. This is necessary since the aromatic hydrocarbon starting materials are not themselves conductive. In addition, the electrolyte should have a higher half-wave potential than the half-wave potential of the aromatic hydrocarbon starting material in order to avoid reduction of the electrolyte in preference to reduction of the aromatic hydrocarbon. The aromatic hydrocarbon starting materials of the invention have rather high reduction potentials, i.e., the half-wave potential is a rather negative voltage. The electrolyte should have a higher reduction potential: its half-wave potential should be a larger negative voltage, i.e., more negative, than that of the aromatic hydrocarbon. For example, a satisfactory electrolyte for use with anthracene, which has a half-wave potential of 2 volts, is tetraethylammonium bromide, which has a half-wave potential of -2.7 volts. Another suitable electrolyte would be triethanolethylammonium bromide, Whichhas a half-wave potential of 2.4 volts.

The half-wave potentials of hydrocarbons and electrolytes are properties readily determinable by known procedures. Considerable information concerning half-wave potentials of various hydrocarbons and electrolytes is contained in K. Schwabe, Polarographie and Chemische Konstitution Organischer Verbindungen (1947). Electrolytes which have the indicated half-wave potential relationship are suitable for use according to the invention, and the criteria for selection are therefore available to a person skilled in the art. Since the tetraalkylammonium halides have half-wave potentials frequently suitable for the present purpose, they are preferred electrolytes.

Tthe cathodic reduction according to the invention is also carried out in the presence of a mutual solvent for both the aromatic hydrocarbon and the electrolyte. The use of the solvent is necessary since the aromatic hydrocarbon starting materials are generally insoluble in suitable electrolytes. The solvent should have a decomposition potential more negative than the half-wave potential of the aromatic hydrocarbon and more negative than the potential employed in the electrolysis, the latter being discussed hereinafter. Decomposition potential is defined in the aforesaid Kirk and Othmer reference and constitutes the potential at which the current begins to turn sharply upward. In the case of a solvent, the decomposition potential is more meaningful than is the half-wave potential since the solvent frequently does not exhibit the typical S-shaped curve that admits of the determination of a halfwave potential. As with half-wave potential, the decomposition potential is negative for cathodic decomposition and positive for anodic decomposition. All decomposition potentials referred to herein are for cathodic decomposition. The decomposition potential of the solvent should be more negative than the half-wave potential of the aromatic hydrocarbon in order to avoid reduction of the solvent in preference to reduction of the aromatic hydrocarbon, and should be more negative than the potential employed in the electrolysis in order to avoid reduction of the solvent simultaneously with reduction of the aromatic hydrocarbon and consequent contamination of the sulfonic acid reaction product with additional reaction products.

Similarly to half-wave potential, decomposition potential is a property which is readily determinable by known procedure, and the criteria for selection of a solvent are therefore available to a person skilled in the art. Solvents which will be suitable for use in most cases include dioxane, aqueous dioxane, acetonitrile, and dimethylformamide. These solvents have decomposition potentials higher, i.e., more negative, than 3 volts which is substantially higher than the half-wave potentials of most of the aromatic hydrocarbon starting materials, most electrolytes, and the potentials which will be employed in most cases. Other solvents having a suitable decomposition potential and the necessary solubility characteristics can also be used.

As described hereinbefore, the electrolysis is also carried out in the presence of S The S0 can be added to the electrolysis medium, i.e., to the solution of the aromatic hydrocarbon and electrolyte in the solvent, in any convenient manner, but it is generally most convenient to merely bubble gaseous S0 into the electrolysis medium. The amount of S0 consumed will vary but will depend primarily upon the rate at which the aromatic hydrocarbon is reduced.

The electrolysis as described above produces a sulfonic acid anion which is then converted to sulfonic acid by reaction in situ with hydrogen ion. See, for example, Equations 3m and 3b hereinbefore. Preferably a source of hydrogen ion is supplied by adding a small amount of water to the electrolysis medium. The addition of water has a further advantage in some cases in that it also increases the solubility of the electrolyte in the solvent. The use of water per se as a source of hydrogen ion is not essential, however, for other compounds such as methanol, dimethylformamide, and acetonitrile will also liberate hydrogen ion under the conditions of the electrolysis' Preferably, however, the electrolysis medium contains 0.525.0 percent water based on the weight of the solvent, the exact amount depending upon the functions the water is to serve. An alternate suitable procedure is to add water or other hydroxyl producing compound to the electrolyzed solution after completion of the electrolysis. In other words, after the solution has been electrolyzed, the potential can be shut off and the sulfonic acid anion can then be reacted with hydrogen ion to produce sulfonic acid.

The potential employed should be greater than the halfwave potential of the aromatic hydrocarbon. This requirement is, of course, inherent in the term cathodic reduction. Also, as described hereinbefore, the potential employed should be less negative than the decomposition potential of the solvent. Preferably the potential employed is not greater than the half-wave potential of the electrolyte. Where monosulfonic acid is desired (see Equations 2a and 3a hereinbefore), the potential is preferably about 0.1 volt higher than the half-wave potential of the aromatic hydrocarbon. Where disulfonic acid is desired (see Equations 2b and 3b hereinbefore), slightly higher potentials should be used. In any event the potential should be greater than the half-wave potential of the aromatic hydrocarbon starting material; otherwise, no cathodic reduction will occur.

During the electrolysis the aromatic hydrocarbon must be present at the surface of the cathode. This requirement also is, of course, inherent in the term cathodic reduction. Preferably, the electrodes are separated by a diaphragm which is permeable to the electrolyte but impermeable to the aromatic hydrocarbon starting material in order to prevent migration of the starting material to the anode and oxidation at that electrode. Conventional diaphragm materials such as porous asbestos and porous Alundum can be used for this purpose.

The temperature of the electrolysis can be room temperature or elevated temperature. Elevated temperatures are desirable in that they reduce the electrical resistance of the electrolysis medium and hence improve the efficiency of the electrolysis, and also in that they increase the solubility of the electrolyte and aromatic hydrocarbon in the solvent. On the other hand, elevated temperatures reduce the amount of S0 in the electrolysis medium and hence may reduce the rates of the chemical reactions involved in the invention. Because of these competing factors, it is desirable to use only moderately elevated temperatures. Preferred temperatures are those in the range of 250 F., though higher or lower temperatures can be employed.

Conducting the electrolysis under elevated pressure is not required but is desirable in order to increase the amount of S0 in the electrolysis medium. The practical upper limit of pressure depends upon the strength of the equipment used. Generally a pressure of S0 in the range of atmospheric to 200 p.s.i.g. should be employed although other pressures can also be used.

A variety of materials can be used as the electrodes. Examples of suitable materials are zinc, lead, tin, mercury, cadmium, etc., as the cathode and platinum, iron, gold, nickel, etc., as anode.

The sulfonic acid produced in the electrolysis can be separated from unreacted hydrocarbon, from electrolyte, and from solvent by any suitable procedure. Frequently a mixture of unreacted hydrocarbon and the sulfonic acid can be separated from the electrolyte and solvent by fractional crystallization from which mixture the sulfonic acid can be separated from unreacted hydrocarbon by either a subsequent fractional crystallization or by extraction with water. Any other suitable procedure can also be employed such as vacuum distillation of the electrolyte and solvent followed by extraction of sulfonic acid from unreacted hydrocarbon with Water.

The following example illustrates the invention more specifically:

A 0.15 molar solution of tetraethylammonium bromide in dimethylformamide, containing 1 percent water based on the amide, is saturated with anthracene. The resulting solution is placed in an electrolytic cell having a mercury cathode and a carbon anode. The solution is maintained at 150 F. while being electrolyzed at a voltage of 2.1 volts, as referred to an auxiliary calomel reference electrode, and while bubbling 50;, into the solution. The electrolysis is continued for 8 hours.

The product is a solution of 9,10-dihydroanthracene- 9,10-disulfonic acid, anthracene, and tetraethylammonium bromide in dimethylformamide. Anthracene and the disulfonic acid are separated from the quaternary salt and the amide by fractional crystallization. The disulfonic acid is separated from the anthracene by extraction with water. The aqueous extract phase containing the disulfonic acid is then cooled to crystallize the disulfonic acid and the latter is separated by filtration.

Generally similar results are obtained with other aromatic hydrocarbons of the type specified herein such as naphthalene, methyl naphthalene, etc. Generally similar results are also obtained when other electrolytes having suitable half-wave potential, such as tetramethylammonium hydroxide, are used, and when other solvents having suitable decomposition potential, such as 20 percent water in dioxane, are used.

I claim:

1. Process for preparing sulfonic acid in an electrolytic cell containing an anode and a cathode which comprises electrolyzing an aromatic hydrocarbon which contains at least two condensed aromatic rings and which contains a condensed aromatic ring having at least two hydrogen atoms attached to nuclear carbon atoms in the presence of sulfur trioxide, in the presence of an electrolyte having a half-wave potential more negative than that of the hydrocarbon and in the presence of a mutual solvent for the hydrocarbon and the electrolyte, the solvent having a decomposition potential more negative than the half-wave potential of the hydrocarbon, said hydrocarbon being present at the surface of said cathode during said electrolyzing, said electrolyzing being at a potential more negative than the half-wave potential of said aromatic hydrocarbon and less negative than the decomposition potential of said solvent, whereby a sulfonic acid anion is produced, and reacting said sulfonic acid anion in situ with hydrogen ion to form a sulfonic acid.

2. Process according to claim 1 wherein said aromatic hydrocarbon is anthracene.

3. Process according to claim 1 wherein said electrolyte is tetraalkylammonium halide.

4. Process according to claim 1 wherein said solvent is acetonitrile.

5. Process according to claim 1 wherein said solvent is dioxane.

6. Process according to claim 1 wherein said electrolyzing is carried out in the presence of water.

7. Process according to claim 1 wherein said solvent is dimethylformamide.

8. Process according to claim 1 wherein said anode and said cathode are separated by a diaphragm which is permeable to said electrolyte and impermeable to said aromatic hydrocarbon.

References Cited by the Examiner UNITED STATES PATENTS 1,804,527 5/31 Dachlover et al. 260-505 JOHN H. MACK, Primary Examiner. 

1. PROCESS FOR PREPARING SULFONIC ACID IN AN ELECTROLYTIC CELL CONTAINING AN ANODE AND A CATHODE WHICH COMPRISES ELECTROLYZING AN AROMATIC HYDROCARBON WHICH CONTAINS AT LEAST TWO CONDENSED AROMATIC RINGS AND WHICH CONTAINS A CONDENSED AROMATIC RING HAVING AT LEAST TWO HYDROGEN ATOMS ATTACHED TO NUCLEAR CARBON ATOMS IN THE PRESENCE OF SULFUR TRIOXIDE, IN THE PRESENCE OF AN ELECTROLYTE HAVING A HALF-WAVE POTENTIAL MORE NEGATIVE THAN THAT OF THE HYDROCARBON AND IN THE PRESENCE OF A MUTUAL SOLVENT FOR THE HYDROCARBON AND THE ELECTROLYTE, THE SOLVENT HAVING A DECOMPOSITION POTENTIAL MORE NEGATIVE THAN THE HALF-WAVE POTENTIAL OF THE HYDROCARBON, SAID HYDROCARBON BEING PRESENT AT THE SURFACE OF SAID CATHODE DURING SAID ELECTROLYZING, SAID ELECTROLYZING BEING AT A POTENTIAL MORE NEGATIVE THAN THE HALF-WAVE POTENTIAL OF SAID AROMATIC HYDROCARBON AND LESS NEGATIVE THAN THE DECOMPOSITION POTENTIAL OF SAID SOLVENT, WHEREBY A SULFONIC ACID ANION IS PRODUCED, AND REACTING SAID SULFONIC ACID ANION IN SITU WITH HYDROGEN ION TO FORM A SULFONIC ACID. 